Function previous_smallest_range 

pub fn previous_smallest_range<T, V, C: IncDecCpCmp<T, V>, R: GetBeginEnd<T>>(
    begin: &T,
    end: &T,
    src: &[R],
    step: &V,
    t: &C,
) -> (T, T)

Given the current begin and end, returns the smallest next range from src.